Recently, attention has been paid to a direct press molding method in which an optical glass element such as a glass lens is press-molded, and which makes it possible to use the molded surface as it is without polishing or the like.
A glass material used in this press molding method is adjusted in composition so that the glass transition temperature thereof decreases, when it is intended to lower the press molding temperature as well as to improve processability. For example, in order to realize a particularly low temperature, it is known that a substantial decrease in the molding temperature can be realized by changing a composition system of glass from a silica glass to a phosphate glass.
However, the use of the phosphate glass results in extremely poor environmental durability. For this problem, even when the surface thereof is protected by forming an optically functional film such as an antireflection film, environmental durability becomes unfavorable in some cases.
Further, not apply only to glass materials for press molding, there are optical glasses include a phosphate glass containing phosphoric acid as a main component in order for giving priority to optical performance, and a fluorophosphate glass further containing fluorine into a phosphate glass. These have been used while having poor environmental durability, since priority is given to the optical performance. However, the poor environmental durability has posed a big problem in practical use.
When the glass materials having poor environmental durability as described above are put into practical use, improvements have been tried by methods in which (1) a dense film of alumina or the like is used as a first layer of an antireflection film to prevent water from entering (for example, see patent document 1), (2) the whole surface of a uncoated lens is coated with a film such as an antireflection film to eliminate an influence of water (for example, see patent document 2), (3) vacuum deposition, using IAD (ion beam assisted deposition), is used as a film-forming method of an antireflection film, instead of using usual vacuum deposition, (4) paying attention to a surface alteration layer of a molded article, a SiO2 film is formed as a first layer on the surface alteration layer to enhance adhesion between a glass and an antireflection film (for example, see patent document 3), and the like. Incidentally, the first layer referred to in the above-mentioned background art examples (1) and (4) is a layer that is in contact with a core glass. The surface alteration layer referred to in the background art example (4) is the alteration layer generated by thermal influences during press molding described in patent document 3.
[Patent Document 1] JP-A-9-159803
[Patent Document 2] JP-A-2-178601
[Patent Document 3] JP-A-1-287501